Seconds Between Two Datetimes

The second is the base unit of time in the International System of Units (SI). It was redefined in 1967 by the 13th General Conference on Weights and Measures (CGPM) as the duration of 9,192,631,770 periods of the radiation emitted by a caesium-133 atom transitioning between the two hyperfine levels of its ground state. This atomic definition replaced the older astronomical definition tied to the ephemeris second of 1956 and the mean solar second before that.

NASA mission clocks count seconds from a designated reference point. The Apollo program used Mission Elapsed Time (MET) counted from launch; the Space Shuttle program continued the practice. The famous Apollo 11 announcement of T-minus 30 seconds before lunar touchdown on 20 July 1969 was a countdown of the second. Modern launches (Falcon 9, Atlas V, SLS) use T-minus seconds for the count and MET seconds after liftoff. This calculator's live ticker uses the same paradigm.

The Unix epoch, chosen at Bell Labs in 1971 by Ken Thompson, fixes time zero at 1970-01-01T00:00:00 UTC. Unix time counts forward in seconds since that moment, ignoring leap seconds. As of 2026 we are approximately 1.77 billion Unix seconds past the epoch. The 32-bit signed Unix timestamp overflows on 19 January 2038 at 03:14:07 UTC - the Year 2038 problem - which is why most modern systems use 64-bit timestamps.

Leap seconds, introduced in 1972 by the International Earth Rotation and Reference Systems Service (IERS), occasionally add an extra second to UTC to keep civil time aligned with astronomical UT1 (rotation-based time). Twenty-seven leap seconds have been inserted between 1972 and 2024. The CGPM voted in 2022 to abolish leap seconds by 2035. This calculator uses JavaScript's Date primitive, which ignores leap seconds entirely - errors of up to 27 seconds for very long spans crossing many leap insertions.

Sub-second precision matters in finance (high-frequency trading measures order latency in microseconds and nanoseconds), networking (TCP retransmits at 200ms RTT thresholds), and scientific instruments (the LIGO gravitational-wave detector samples at 16,384 Hz, one sample every 61 microseconds). For civil time and elapsed wall-clock duration, the second is the standard unit and this calculator returns whole seconds.

Sport timing has driven athletic-grade second precision. Olympic swimming and athletics resolve to one-hundredth of a second (0.01 s). Formula 1 timing uses one-thousandth (0.001 s). World records under the International Association of Athletics Federations (now World Athletics) require automatic timing to the hundredth of a second since 1977. This page does not handle sub-second resolution; for hundredths use a stopwatch.

Telecommunications meters measure call duration in seconds. Mobile carriers historically billed by the minute (rounded up) but most modern plans either bill by the second or include unlimited talk. The 60-second meter on a payphone (cost per second multiplied by elapsed seconds) was a fixture of pre-2000 public-phone culture. This calculator's live ticker mirrors that experience - one count per second, in real time.